In U.S. Pat. No. 6,483,109 B1, a multiple stage mass spectrometer has been presented with which it is possible to selectively enrich and measure the quantity of predetermined ion species from a large number of stored ions. Increased precision of the m/z filtering and the option to fragment into daughter ions or even granddaughter ions mean that the selectivity of the enrichment is increased to such a degree that a practically certain determination of the desired ion species is achieved, even if ion species of the same mass are present in the initial quantity of the ions.
As the multiple stage mass spectrometer always returns unused ions to the ion storage device, termed “accumulator” below for greater clarity, it is possible to measure the quantity of a larger number of predetermined ion species sequentially from a single sample with the multiple stage mass spectrometer.
This multiple stage mass spectrometer is mainly of interest for the quantitative search for specific forms of mutations or modifications or specific expression anomalies of peptides and proteins, but there are many additional fields of application over and above these, for example the determination of the quantity of breakdown products of medicines (metabolites) as a function of time after the medicines have been administered, or the search for specific toxic substances in mixtures of substances.
The analyzing part of the multiple stage mass spectrometer will be termed “molecular analyzer” below. The molecular analyzer requires around one to three seconds to analyze one ion species. If other ion species, for example other daughter or granddaughter ions, are measured for the same substance by way of confirmation, this can be done in a further one to two seconds. Some two to five seconds are therefore required for the certain determination of a molecular species. If 20 to 100 molecule species are to be measured with double reliability measurement, this requires between one and eight minutes; on average around two to four minutes.
The accumulator should be able to collect around 108 to 109 ions in order to achieve a high detection sensitivity. The filling can preferably be undertaken by nanospraying from a thin capillary with an extended tip. This type of electrospray ionization is the most efficient for transferring the analyte molecules used into an accumulator located in a vacuum. It is possible to feed several 105, up to a maximum of around 106, ions per second into an accumulator in the vacuum. To fill the accumulator with at least 108 ions therefore requires at least 100 seconds, in practice, however, more like three to six minutes or more are required. A complete analytical process with filling of the accumulator and subsequent analysis is performed in about five to ten minutes. The heart of the instrument, the molecular analyzer, is utilized quite inefficiently in this situation with a two to four minute share of the time.
The molecular analyzer consists of a chain of linear quadrupole cells operated in unison. It is very slim: at a length of around 30 centimeters without the accumulator and without the ion detector, a maximum diameter of around five to ten centimeters, including the voltage feeders and supports, is possible. With accumulator and ion detector one must expect a length of around 50 centimeters. If one also includes the nanospray device with its inlet capillary and at least one differential pump pressure stage, it is easy to end up with a somewhat unwieldy length of around 80 to 90 centimeters for the whole instrument, which is otherwise very compact.